Honor legacy basic authentication encoding property - #4358

Fixes #4347

Problem

BasicAuthorizationProvider reads logging.auth.encoding with an overload that applies the platform default charset. When that property is absent, it also discarded the fallback lookup for the Spring-compatible property.

Solution

• Read the legacy property without an implicit platform-default fallback.
• Assign the Spring-compatible property lookup result so UTF-8 is used when neither property is configured.
• Add a regression test covering non-ASCII credentials and the legacy encoding property.
• Add the required 2.x changelog entry.

Testing

• ./mvnw -pl :log4j-core-test -am -Dtest=BasicAuthorizationProviderTest -Dsurefire.failIfNoSpecifiedTests=false -Dspotless.check.skip=true -Dspotless.apply.skip=true -Drat.skip=true -Dcheckstyle.skip=true test — passed
• git diff --check — passed